Hi Andreas,
As of now i am unable to access your site. So can’t say what the issue is. However there are mainly 2 cases when your lower case text are converted to uppercase.
1. The font doesn’t support the lower case characters. Please check the demo of the font to confirm this. You should get the demo from where you have downloaded the font. In this case, you should use the font that supports the lowercase characters
2. You or your theme might have used the css to make the text to show in uppercase. In this case, you need to remove the text-transform propertycss definition. For details about the this property you can check this documentation. http://www.w3schools.com/cssref/pr_text_text-transform.asp
Thanks